Lithium transition metal oxide powders and, in particular, lithium cobaltic dioxide, form key cathodic materials for the positive electrode (cathode) in rechargeable lithium ion electrochemical cells.
Specific physical, morphological and chemical characteristics are required to sustain the transition metal oxide's performance over several hundred sequential charge and discharge cycles demanded during service. Current battery applications for the powder demand high purity, homogeneity, controlled particle size (1 micron to 50 microns) and low surface area (less than 2.0 m2/g).
Selection of the lithiated transition metal oxides, or indeed any cathodic material, is dependant upon the material having a high reversible capacity and conversely a low irreversible capacity, high thermal stability and low cost. Of the three most commonly contemplated compounds, lithium cobaltic dioxide exhibits a high capacity concomitant with a good thermal stability, but it is extremely costly. Lithium nickel dioxide possesses high capacity, with low relative cost but is thermally unstable. Spinel lithium manganese oxide (LiMn2O4) is the most thermally stable of the three, and is relatively inexpensive, but lacks a high capacity.
The literature abounds in examples of novel lithium ion systems and variations on the methods for the preparation thereof. In U.S. Pat. No. 4,302,518 issued to J. B. Goodenough et al., lithium cobalt dioxide is prepared by calcining a pelletized mixture of lithium and cobalt carbonates in air at 900° C. for several hours. The calcining step may be repeated one or more times to ensure complete conversion to the desired product. The resultant lithiated cobalt dioxide is characterized in having a hexagonal structure with lattice constants a=0.282 nm and c=1.408 nm as described by T. Ohzuku et al. (J. Electrochem. Soc. 141, 2972, 1994). Reaction parameters will determine lattice structures. Thus, as disclosed in Solid State Ionics, 53-56, 681 (1992) by R. J. Gummow et al., lithium cobalt dioxide prepared by the reaction of lithium and cobalt carbonates in air at 400° C. for between 2 to 5 days yields a product having a cubic structure having the lattice constant a=0.28297 nm (c/a=4.90).
In U.S. Pat. No. 4,980,080 issued to A. Lecerf et al., there is described a process for the synthesis of LiyNi2-yO2 or LiNi1-xCoxO2. A mixture of hydrated lithium hydroxide and nickel and cobalt oxide are heated in air at temperatures ranging between 600° C. to 800° C. A reheating step is then undertaken to complete the solid state reaction.
U.S. Pat. No. 5,264,201 to J. R. Dahn et al. teaches a process for the production of lithium nickel dioxide involving the reaction of nickel oxide or nickel hydroxide, with an excess of lithium hydroxide, and heating the mixture above 600° C. in an atmosphere substantially free of carbon dioxide and with a high partial pressure ratio of oxygen to water.
Furthermore, it has been suggested that solid solutions of LiNi1-xCoxO2 and LiNi1-xAlxO2 be used in rechargeable lithium battery applications. The article by T. Ohzuku et al., J. Electrochem. Soc. 142, 4033 (1995) states that LiNi3/4Al1/4O2 can be made by reacting lithium nitrate, nickel carbonate and aluminium hydroxide at 750° C. under an oxygen atmosphere for 20 hours. Advantageously, the solid state reaction product exhibits a higher stability than lithium nickel dioxide per se but deleteriously has a lower rechargeable capacity. The minimum cobalt content in the LiNi1-xCoxO2 compound required to improve stability is significant, and thus is expensive.
Lithium transition metal oxides presently used in rechargeable lithium batteries can exhibit one of two common problems namely, poor thermal stability, or a high fade rate, particularly when cycling at a reversible capacity above about 150 mAh/g. There is a preference for lithium cathode materials that demonstrate good stability when cycling at a reversible capacity of about 200 mAh/g at an average discharge voltage above 3.0 volts.
Thus all manufacturers of lithium ion battery systems attempt to attain the highest reversible capacity commensurate with safe operation. Battery design must take into account the potential dangers of fire or explosion caused by oxygen release from the lithium transition metal oxide within the battery, particularly at the time of its highest charge. Damaging reactions can occur due to increases in temperature when the cell is shorted out or misused.